Production of hydrocarbons

ABSTRACT

A process for production of hydrocarbons including a) reforming a divided hydrocarbon feedstock stream, mixing the first stream with steam, passing the mixture over a catalyst disposed in heated heat exchange reformer tubes to form a primary reformed gas, forming a secondary reformer feed stream including the primary reformed gas and the second hydrocarbon stream, partially combusting the secondary reformer feed stream and bringing the partially combusted gas towards equilibrium over a secondary catalyst, and producing a partially cooled reformed gas, b) further cooling the partially cooled reformed gas below the dew point of steam therein to condense water and separating condensed water to give a de-watered synthesis gas, c) synthesising hydrocarbons from the de-watered synthesis gas by the Fischer-Tropsch reaction and separating some of the synthesised hydrocarbons into a tail gas, and d) incorporating part of the tail gas into the secondary reformer feed stream before partial combustion thereof.

This application is the U.S. national phase application of PCTInternational Application No. PCT/GB2003/004622, filed Oct. 23, 2003,and claims priority of British Patent Application No. 0225961.2, filedNov. 7, 2002.

FIELD OF THE INVENTION

This invention relates to the production of hydrocarbons by theFischer-Tropsch process and to the production of synthesis gas therefor.The synthesis gas contains hydrogen and carbon oxides and is produced bythe catalytic reaction of steam with a hydrocarbon feedstock in aprocess known as steam reforming.

BACKGROUND OF THE INVENTION

Steam reforming is widely practised and is used to produce hydrogenstreams and synthesis gas for a number of processes such as ammonia,methanol as well as the Fischer-Tropsch process. In a steam reformingprocess, a desulphurised hydrocarbon feedstock, e.g. methane, naturalgas or naphtha, is mixed with steam and passed at elevated temperatureand pressure over a suitable catalyst, generally a transition metal,especially nickel, on a suitable support. Methane reacts with steam toproduce hydrogen and carbon oxides. Any hydrocarbons containing two ormore carbon atoms that are present are converted to carbon monoxide andhydrogen, and in addition, the reversible methane/steam reforming andshift reactions occur. The extent to which these reversible reactionsproceed depends upon the reaction conditions, e.g. temperature andpressure, the feed composition and the activity of the reformingcatalyst. The methane/steam reforming reaction is highly endothermic andso the conversion of methane to carbon oxides is favoured by hightemperatures. For this reason, steam reforming is usually effected atoutlet temperatures above about 600° C., typically in the range 650° C.to 950° C., by passing the feedstock/steam mixture over a primary steamreforming catalyst disposed in externally heated tubes. The compositionof the product gas depends on, inter alia, the proportions of thefeedstock components, the pressure and temperature. The product normallycontains methane, hydrogen, carbon oxides, steam and any gas, such asnitrogen, that is present in the feed and which is inert under theconditions employed. For applications such as Fischer-Tropsch synthesis,it is desired that the molar ratio of hydrogen to carbon monoxide isabout 2 and the amount of carbon dioxide present is small.

In order to obtain a synthesis gas more suited to Fischer-Tropschsynthesis, the primary reformed gas may be subjected to secondaryreforming by partially combusting the primary reformed gas using asuitable oxidant, e.g. air or oxygen. This increases the temperature ofthe reformed gas, which is then passed adiabatically through a bed of asecondary reforming catalyst, again usually nickel on a suitablesupport, to bring the gas composition towards equilibrium. Secondaryreforming serves three purposes: the increased temperature resultingfrom the partial combustion and subsequent adiabatic reforming resultsin a greater amount of reforming so that the secondary reformed gascontains a decreased proportion of residual methane. Secondly theincreased temperature favours the reverse shift reaction so that thecarbon monoxide to carbon dioxide ratio is increased. Thirdly thepartial combustion effectively consumes some of the hydrogen present inthe reformed gas, thus decreasing the hydrogen to carbon oxides ratio.In combination, these factors render the secondary reformed gas formedfrom natural gas as a feedstock more suited for use as synthesis gas forapplications such as Fischer-Tropsch synthesis than if the secondaryreforming step was omitted. Also more high grade heat can be recoveredfrom the secondary reformed gas: in particular, the recovered heat canbe used to heat the catalyst-containing tubes of the primary reformer.Thus the primary reforming may be effected in a heat exchange reformerin which the catalyst-containing reformer tubes are heated by thesecondary reformed gas. Examples of such reformers and processesutilising the same are disclosed in for example U.S. Pat. No. 4,690,690and U.S. Pat. No. 4,695,442.

WO 00/09441 describes a process wherein a feedstock/steam mixture issubjected to primary reforming over a catalyst disposed in heated tubesin a heat exchange reformer, the resultant primary reformed gas issubjected to secondary reforming by partially combusting the primaryreformed gas with an oxygen-containing gas, the resultant partiallycombusted gas then being brought towards equilibrium over a secondaryreforming catalyst, and the resultant secondary reformed gas used toheat the tubes of the heat exchange reformer. In the process, nohydrocarbon feedstock by-passes the primary reforming stage. Carbondioxide is separated from the secondary reformed gas before or after itsuse for the synthesis of carbon containing compounds, and is recycled tothe primary reformer feed. In one embodiment described in WO 00/09441,the recycled carbon dioxide is part of the tail gas from aFischer-Tropsch synthesis process, and is added to the natural gasfeedstock prior to desulphurisation of the latter.

Fischer-Tropsch tail gas is liable to contain a significant amount ofcarbon monoxide. If this is added to the feedstock prior to primaryreforming in a heat exchange reformer, the carbon monoxide undergoes theexothermic methanation reaction resulting in a faster increase intemperature of the gas undergoing reforming than if the tail gas had notbeen added. The temperature difference between the gas undergoingreforming and the heating medium is thus decreased and so more heattransfer area, e.g. more and/or longer heat exchange tubes, is requiredfor a given reforming duty.

In our co-pending application PCT/GB 02/03311 we have demonstrated thatthis problem may be overcome by addition of the Fischer-Tropsch tail gasto the primary reformed gas before partial combustion thereof, i.e.addition of tail gas to the primary reformed gas between the steps ofprimary and secondary reforming. Such addition, where carbon dioxide ispresent in the tail gas or is added from another source, further has theeffect of allowing lower steam ratios to be used in the primaryreformer. [By the term “steam ratio” we mean the ratio of the number ofmoles of steam to the number of gram atoms of hydrocarbon carbon in thefeed: thus a methane/steam mixture comprising 2 moles of steam per moleof methane has a steam ratio of 2.] This has advantages in respect ofproviding lower operating costs, for example in steam generation.

Use of lower steam ratios, for example steam ratios below 1.00, can,however, lead to carbon formation on the exposed surfaces of thecatalyst. Such carbon formation has the undesirable effect of increasingthe pressure drop through the catalyst. It can also result in a loss ofcatalyst activity. Thus there is a desire to use lower steam ratios thanthose previously achieved without the risk of increasing carbondeposition.

SUMMARY OF THE INVENTION

We have found that operation at low overall steam ratios with economicalreforming of the hydrocarbon feedstock may be achieved by dividing thefeedstock into two streams, mixing the first stream with steam andfeeding it to the primary reformer and feeding the second stream to theprimary reformed gas before secondary reforming along with at least partof the tail gas from the Fischer-Tropsch process. The steam ratio istherefore lower overall but still sufficiently high in the primaryreforming step to avoid carbon deposition.

Accordingly the present invention provides a process for the productionof hydrocarbons comprising;

-   a) subjecting a hydrocarbon feedstock to steam reforming by    -   i) dividing the feedstock into first and second streams,    -   ii) mixing the first stream with steam, passing the mixture of        the first stream and steam over a catalyst disposed in heated        tubes in a heat exchange reformer to form a primary reformed        gas,    -   iii) forming a secondary reformer feed stream comprising the        primary reformed gas and the second hydrocarbon stream,    -   iv) partially combusting the secondary reformer feed stream with        an oxygen-containing gas and bringing the resultant partially        combusted gas towards equilibrium over a secondary reforming        catalyst, and    -   v) using the resultant secondary reformed gas to heat the tubes        of the heat exchange reformer, thereby producing a partially        cooled reformed gas,-   b) further cooling the partially cooled reformed gas to below the    dew point of the steam therein to condense water and separating    condensed water to give a de-watered synthesis gas,-   c) synthesising hydrocarbons from said de-watered synthesis gas by    the Fischer-Tropsch reaction and separating at least some of the    synthesised hydrocarbons, to give a tail gas, and-   d) incorporating at least part of said tail gas into the secondary    reformer feed stream before the partial combustion of thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated by reference to the accompanying drawingsin which;

FIG. 1 is a schematic of one embodiment of the invention wherebyFischer-Tropsch tail gas and hydrocarbon feedstock are added separatelyto the primary reformed gas to form the secondary reformer feed stream,

FIG. 2 is a schematic of a second embodiment of the invention where, inaddition to Fischer-Tropsch tail gas and hydrocarbon, carbon dioxideseparated from secondary reformed gas is added to the primary reformedgas to form the secondary reformer feed stream,

FIG. 3 is a schematic of a third embodiment of the invention wherebyFischer-Tropsch tail gas and hydrocarbon feedstock are combined, heatedand added to the primary reformed gas to form the secondary reformerfeed stream.

FIG. 4 is a table that contains data calculated for a Fischer-Tropschprocess operated in accordance with the schematic depicted in FIG. 2.

FIG. 5 is a table that contains data calculated for a 80,000barrel-per-day Fischer Tropsch process operated in accordance with theschematic depicted in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, the primary reforming is effected using a heatexchange reformer. In one type of heat exchange reformer, the catalystis disposed in tubes extending between a pair of tube sheets through aheat exchange zone. Reactants are fed to a zone above the upper tubesheet and pass through the tubes and into a zone beneath the lower tubesheet. The heating medium is passed through the zone between the twotube sheets. Heat exchange reformers of this type are described in GB 1578 270 and WO 97/05 947.

Another type of heat exchange reformer that may be used is a double-tubeheat exchange reformer as described in U.S. Pat. No. 4,910,228 whereinthe reformer tubes each comprise an outer tube having a closed end andan inner tube disposed concentrically within the outer tube andcommunicating with the annular space between the inner and outer tubesat the closed end of the outer tube with the steam reforming catalystdisposed in said annular space. The external surface of the outer tubesis heated by the secondary reformed gas. The mixture of hydrocarbonfeedstock, carbon dioxide and steam is fed to the end of the outer tubesremote from said closed end so that the mixture passes through saidannular space and undergoes steam reforming and then passes through theinner tube. As in the double-tube reformer of U.S. Pat. No. 4,910,228,in the present invention preferably insulation is provided on the wallsof the inner tube.

In the process of the invention the feedstock may be any gaseous or lowboiling hydrocarbon feedstock such as natural gas or naphtha. It ispreferably methane or natural gas containing a substantial proportion,e.g. over 90% v/v methane. If the feedstock contains sulphur compounds,before, or preferably after, compression, but before the feedstock isdivided, the feedstock is subjected to desulphurisation, e.g.hydrodesulphurisation and absorption of hydrogen sulphide using asuitable absorbent, e.g. a zinc oxide bed. The feedstock is typicallycompressed to a pressure in the range 10-100 bar abs, particularly 20-60bar abs.

Before, or preferably after, compression of the feedstock, the feedstockis divided into two streams. The first stream is mixed with steam: thissteam introduction may be effected by direct injection of steam and/orby saturation of the feedstock by contact of the latter with a stream ofheated water. The amount of steam introduced is such as to give anoverall steam ratio of 0.5 to 2, preferably 1 to 2, i.e. 0.5 to 2,preferably 1 to 2 moles of steam per gram atom of hydrocarbon carbon inthe feedstock. The steam ratios that may be employed in the processpresent invention without carbon deposition may be affected by thechoice of primary steam reforming catalyst. Typically lower steam ratiosmay be used when the primary steam reforming catalyst is a preciousmetal-based catalyst compared to nickel-based catalysts. The amount ofsteam is preferably minimised as this leads to a lower cost, moreefficient process. It is preferred that the steam ratio is below 1.5,more preferably below 1.0. When a steam ratio below 1.0 is used it ispreferable that at least a portion of the primary steam reformingcatalyst is a precious metal catalyst.

The resultant feedstock/steam mixture is then subjected to reforming.Before it is fed to the heat exchange reformer, the feedstock/steammixture may be subjected to a step of adiabatic low temperaturereforming. In such a process, the hydrocarbon/steam mixture is heated,typically to a temperature in the range 400-650° C., and then passedadiabatically through a bed of a suitable catalyst, usually a catalysthaving a high nickel content, for example above 40% by weight. Duringsuch an adiabatic low temperature reforming step any hydrocarbons higherthan methane react with steam to give a mixture of methane, carbonoxides and hydrogen. The use of such an adiabatic reforming step,commonly termed pre-reforming, is desirable to ensure that the feed tothe heat exchange reformer contains no hydrocarbons higher than methaneand also contains a significant amount of hydrogen. This is desirable inorder to minimise the risk of carbon formation on the catalyst in theheat-exchange reformer.

After any such pre-reforming step, the mixture is further heated, ifnecessary, to the heat exchange reformer inlet temperature, which istypically in the range 300-500° C. During passage through the reformingcatalyst, the endothermic reforming reaction takes place with the heatrequired for the reaction being supplied from the secondary reformed gasflowing past the exterior surface of the outer tubes. The primaryreforming catalyst may be nickel supported on a refractory support suchas rings or pellets of calcium aluminate cement, alumina, titania,zirconia and the like. Alternatively, particularly when a steam ratioless than 1.0 is employed, a precious metal catalyst may be used as theprimary reforming catalyst. Suitable precious metal catalysts includerhodium, ruthenium and platinum between 0.01 and 2% by weight on asuitable refractory support such as those used for nickel catalysts.Alternatively a combination of a nickel and precious metal catalyst maybe used. For example, a portion of the nickel catalyst may be replacedwith a precious metal catalyst, such as a ruthenium-based catalyst.

The temperature of the secondary reformed gas is preferably sufficientthat the gas undergoing primary reforming leaves the catalyst at atemperature in the range 650-850° C.

In the present invention a proportion of the total hydrocarbon feedstockfed to the process (the second stream) bypasses the primary reformingstep and is combined with the primary reformed gas to form a secondaryreformer feed stream which is then subjected to partial combustion in asecondary reforming step. The resulting secondary reformed gas isde-watered and used as the synthesis gas for the Fischer-Tropschsynthesis of hydrocarbons. A tail gas from the Fischer-Tropsch synthesisis recycled to the secondary reformer feed stream. In forming thesecondary reformer feed stream the Fischer-Tropsch tail gas and secondhydrocarbon stream may be added separately in any order to the primaryreformed gas or may be pre-mixed if desired before being fed to theprimary reformed gas. Pre-mixing the tail gas and second hydrocarbonstream has the advantage that, if necessary, they may be heated togetherin one rather than two heat exchangers. Howsoever the second hydrocarbonstream and the Fischer-Tropsch tail gas are added it is preferable, toavoid decomposition of the hydrocarbons therein, that they are notheated to temperatures in excess of 420° C. prior to combination withthe primary reformed gas.

The secondary reformer feed stream comprising the primary reformedgas/hydrocarbon/tail gas mixture is then subjected to secondaryreforming by adding a gas containing free oxygen, effecting partialcombustion and passing the partially combusted gas through a secondaryreforming catalyst. Whereas some steam may be added to the oxygencontaining gas, preferably no steam is added so that the low overallsteam ratio for the reforming process is achieved. The secondaryreforming catalyst is usually nickel supported on a refractory supportsuch as rings or pellets of calcium aluminate cement, alumina, titania,zirconia and the like. The gas containing free oxygen is preferablysubstantially pure oxygen, e.g. oxygen containing less than 1% nitrogen.However where the presence of substantial amounts of inerts ispermissible, the gas containing free oxygen may be air or enriched air.Where the gas containing free oxygen is substantially pure oxygen, formetallurgical reasons it is preferably fed to the secondary reformer ata temperature below about 250° C.

The amount of oxygen required in the secondary reformer is determined bytwo main considerations, viz. the desired composition of the productgas, and the heat balance of the heat exchange reformer. In general,increasing the amount of oxygen, thereby increasing the temperature ofthe reformed gas leaving the secondary reformer, causes the [H₂]/[CO]ratio to decrease and the proportion of carbon dioxide to decrease.Alternatively, if the conditions are arranged such that the temperatureis kept constant, increasing the temperature at which the feedstock isfed to the heat exchange reformer decreases the amount of oxygenrequired (at a constant oxygen feed temperature). Decreasing therequired amount of oxygen is advantageous as this means that a smaller,and hence cheaper, air separation plant can be employed to produce theoxygen. The temperature of the feedstock can be increased by anysuitable heat source, which may, if necessary, be a fired heater, whichof course can use air, rather than oxygen, for the combustion.

The amount of oxygen-containing gas added is preferably such that 40 to60 moles of oxygen are added per 100 gram atoms of hydrocarbon feedstockfed to the primary and secondary reforming stages. Preferably the amountof oxygen added is such that the secondary reformed gas leaves thesecondary reforming catalyst at a temperature in the range 800-1050° C.For a given feedstock/steam mixture, amount and composition of theoxygen-containing gas and reforming pressure, this temperature largelydetermines the composition of the secondary reformed gas.

The secondary reformed gas is then used to provide the heat required forthe primary reforming step by using the secondary reformed gas as thehot gas flowing past the tubes of the heat exchange reformer. Duringthis heat exchange the secondary reformed gas cools by transferring heatto the gas undergoing primary reforming. Preferably the secondaryreformed gas cools by several hundred ° C. but of course it will leavethe heat exchange reformer at a temperature somewhat above thetemperature at which the feedstock/steam/carbon dioxide mixture is fedto the heat exchange reformer. Preferably the secondary reformed gasleaves the heat exchange reformer at a temperature in the range 500-650°C.

After leaving the heat exchange reformer, the secondary reformed gas isthen further cooled. Heat recovered during this cooling may be employedfor reactants pre-heating and/or for heating water used to provide thesteam employed in the primary reforming step. As described hereinafter,the recovered heat may additionally, or alternatively, be used in acarbon dioxide separation step.

The secondary reformed gas is cooled to a temperature below the dewpoint of the steam in the secondary reformed gas so that the steamcondenses. The condensed steam is then separated. The cooling to effectcondensation of the steam may be effected by contacting the secondaryreformed gas with a stream of cold water: as a result a stream of heatedwater is formed which may be used to supply some or all of the steamrequired for reforming.

Typically the secondary reformed gas contains 5 to 15% by volume ofcarbon dioxide (on a dry basis). In one embodiment of the invention,after separation of the condensed water, carbon dioxide is separatedfrom the synthesis gas prior to the Fischer-Tropsch synthesis stage andrecycled to the synthesis gas production. Such recycle of carbon dioxideis preferred as it provides a means to control [H₂]/[CO] ratio toachieve the optimal figure for FT synthesis of about 2. Preferably theamount of recycled carbon dioxide is maximised up to the quantity whichis needed to achieve this ratio. Typically this may be at least 75%,particularly at least 90%, of the carbon dioxide in the de-wateredsecondary reformed gas. The recycled carbon dioxide stream may be added,as in the aforesaid WO 00/09441, to the feedstock prior to feeding thelatter to the heat exchange reformer or preferably to the secondaryreformer feed stream before the latter is fed to the secondary reformingstep. The carbon dioxide may be added before, after or together with thehydrocarbon feedstock and tail gas. Preferably the recycled carbondioxide is added separately to the secondary reformer feed streambecause it may be heated to temperatures greater than 420° C. As statedabove, where the recycled carbon dioxide (either as carbon dioxideseparated from the synthesis gas prior to synthesis and recycled, or asthe recycled tail gas) is added to the primary reformed gas, rather thanto the feedstock prior to primary reforming, there is an advantage inthat the primary reforming process can be operated at a lower steamratio.

The carbon dioxide may be separated by a conventional “wet” process oralternatively a pressure swing adsorption process may be employed. In aconventional “wet” process the secondary reformed gas is de-watered andis then contacted with a stream of a suitable absorbent liquid, such asan amine, particularly methyl diethanolamine (MDEA) solution so that thecarbon dioxide is absorbed by the liquid to give a laden absorbentliquid and a gas stream having a decreased content of carbon dioxide.The laden absorbent liquid is then regenerated, for example by heating,to desorb the carbon dioxide and to give a regenerated absorbent liquid,which is then recycled to the carbon disoxide absorption stage. At leastpart of the desorbed carbon dioxide is recycled to the primary reformingstep as described above. If the carbon dioxide separation step isoperated as a single pressure process, i.e. essentially the samepressure is employed in the absorption and regeneration steps, only alittle recompression of the recycled carbon dioxide will be required.Unless it is desired that the product synthesis gas has a very lowcarbon dioxide content, it is generally not necessary to effect theregeneration of the absorbent liquid to a very low carbon dioxidecontent.

Alternatively, or in addition to a stage of carbon dioxide separationand recycle, before the de-watered synthesis gas is passed to theFischer-Tropsch hydrocarbon synthesis stage it may be further subjectedto a step of hydrogen separation, e.g. through a membrane in order toprovide pure hydrogen for other uses e.g. hydrocracking orhydrodesulphurisation. In this situation, the tail gas recycle (in theabsence of carbon dioxide separation and recycle) or the carbon dioxiderecycle stream are controlled to give a [H₂]/[CO] ratio, which is higherthan the optimum for Fischer-Tropsch synthesis, so that after therequired amount of hydrogen is separated the resulting synthesis gas hasan [H₂]/[CO] ratio of about 2.

In the Fischer-Tropsch process, a synthesis gas containing carbonmonoxide and hydrogen is reacted in the presence of a catalyst, which istypically a cobalt- and/or iron-containing composition. The process maybe effected using one or more fixed catalyst beds or one or morereactors using a moving catalyst, for example a slurry of the catalystin a hydrocarbon liquid. The product hydrocarbon liquid is separatedfrom the residual gas. The reaction may be carried out in a single passor part of the residual gas may be combined with fresh synthesis gas andrecycled to the Fischer-Tropsch reactor. Any residual gas which is notrecycled to the Fischer-Tropsch reactor for further reaction is hereintermed tail gas. Since the reaction of the synthesis gas is incomplete,the tail gas will contain some hydrogen and carbon monoxide. Inaddition, the tail gas may also contain some light hydrocarbons, e.g.paraffins including methane, ethane, butane, olefins such as propylene,alcohols such as ethanol, and traces of other minor components such asorganic acids. It will generally also contain some carbon dioxide, whichmay be present in the synthesis gas fed to the Fischer-Tropsch reactionand/or is formed by side reactions. Possibly, as a result of incompleteseparation of the liquid hydrocarbon product, the tail gas may alsocontain a small proportion of higher hydrocarbons, i.e. hydrocarbonscontaining 5 or more carbon atoms. These components of the tail gasrepresent a valuable source of carbon and hydrogen.

In the present invention at least part of the tail gas is recycled andused as part of the feedstock employed to make the Fischer-Tropschsynthesis gas. The amount of tail gas recycled is preferably between 5and 100% by volume of the tail gas produced in the Fischer-Tropschsynthesis stage.

In the present invention the hydrocarbon feedstock is divided into twostreams. The second hydrocarbon stream bypasses the primary reformingstep and is added to the secondary reformer feed stream prior tocombustion thereof in the secondary reformer. The second hydrocarbonstream comprises between 5 and 50% by volume, preferably between 5 and40% by volume and most preferably between 5 and 30% by volume of thehydrocarbon feedstock. Amounts less than 5% by volume provide too smalla benefit whereas amounts greater than 30%, especially 50% are lesseconomically attractive due to a consequential increase in size and costof the primary reformer (because of the resulting drop in the secondaryreformed gas temperature and thereby heat exchange with the primaryreformer), or increased requirement for oxygen in the secondaryreforming step.

By providing a proportion of the hydrocarbon feedstock and at least partof the Fischer-Tropsch tail gas to the primary reformed gas, it ispossible to operate the process at low overall steam ratios without therisk of carbon deposition. Overall steam ratios in the range 0.8 to 1.2may be achieved using the process of the present invention without therisk of significant carbon deposition in the primary reforming stage.

Referring now to the drawing. in FIG. 1, hydrocarbon feedstock, forexample natural gas containing over 90% v/v methane, fed via line 10 isdivided into two streams. The first stream is fed via line 12 to asaturator 14 where it is contacted with hot water provided by line 16.Waste hot water is recovered via line 18 and may be recycled if desired.The resulting mixture of first hydrocarbon stream and steam is fed,typically at a pressure in the range 10 to 60 bar abs., via line 20 to aheat exchanger 22 and thence, via line 24, to the catalyst-containingtubes 26 of a heat exchange reformer 28. The mixture is typically heatedto a temperature in the range 300 to 500°C. prior to entry into thetubes 26. For simplicity only one tube is shown in the drawing: inpractice there may be several tens or hundreds of such tubes.

The feedstock/steam mixture undergoes primary steam reforming in thetubes 26 and the primary reformed gas leaves the heat exchange reformer28 via line 30, typically at a temperature in the range 650 to 850° C.

The primary reformed gas in line 30 is mixed with Fischer-Tropsch tailgas (to be described) fed via line 32. The resulting primary reformedgas/tail gas mixture then proceeds via line 34 and is mixed with thesecond hydrocarbon stream, fed via line 36 and which has been pre-heatedin heat exchanger 38. The resulting secondary reformer feed streamcomprising the primary reformed gas/tail gas/hydrocarbon mixture is fedvia line 40 to a secondary reformer 42, to which oxygen is supplied vialine 44.

The secondary reformer feed stream is partially combusted in thesecondary reformer and brought towards equilibrium by passage over asecondary reforming catalyst. The secondary reformed gas leavessecondary reformer via line 46, typically at a temperature in the range900 to 1050° C.

Heat is recovered from the hot secondary reformed gas by passing thesecondary reformed gas via line 46 to the shell side of the heatexchange reformer 28 so that the secondary reformed gas forms theheating medium of the heat exchange reformer. The secondary reformed gasis thus cooled by heat exchange with the gas undergoing reforming in thetubes 26 and leaves the heat exchange reformer via line 48, typically ata temperature 50 to 200° C. above the temperature at which the firsthydrocarbon stream/steam mixture is fed to the tubes 26.

The partially cooled secondary reformed gas is then cooled further withheat recovery in one or more heat exchangers 50 to a temperature belowthe dew point of the water in the secondary reformed gas. The cooledsecondary reformed gas is then fed via line 52 to a separator 54 whereincondensed water is separated as a liquid water stream 56. This water maybe recycled by heating it in a heat exchanger (not shown) and feeding itto line 16 for use in the saturator 14.

The resulting de-watered synthesis gas is then fed from the separator54, via line 58, to an optional hydrogen separation unit 60, e.g. amembrane unit or a pressure swing adsorption stage, to separate part ofthe hydrogen in the de-watered synthesis gas as a hydrogen stream 62.The resultant synthesis gas is then fed via line 64 to a Fischer-Tropschsynthesis stage 66, wherein liquid hydrocarbons are synthesised and areseparated, together with by-product water, as a product stream 68leaving a tail gas stream 70. Part of the tail gas is purged as stream72 to avoid a build up of inerts, e.g. nitrogen which may be present inthe hydrocarbon feedstock or oxygen-containing gas fed to the secondaryreformer. The purged tail gas may be used as fuel, for example in afired heater used for heating the mixture of first hydrocarbon streamand steam fed to the heat exchange reformer. The remainder of the tailgas is fed to a compressor 74 and then to a heat exchanger 76 and thenfed via line 32 to be mixed with the primary reformed gas 30.

In FIG. 2, the second hydrocarbon stream by-passing the primaryreforming stage via line 36 and heat exchanger 38 is mixed with theFischer-Tropsch tail gas fed vial line 32 and the resulting mixture fedvia line 78 to the primary reformed gas 30 to form a primary reformedgas mixture 80.

The de-watered synthesis gas is fed via line 58 to a carbon dioxideseparation stage 82 wherein carbon dioxide is separated from thede-watered synthesis gas. The resulting de-watered, carbondioxide-depleted synthesis gas is fed via line 84 to the optionalhydrogen separation unit 60 and thence to the Fischer-Tropsch synthesisstage 66. The separated carbon dioxide from separation stage 82 is fedvia line 86 to a compressor 88 and then via line 90 to a heat exchanger92. To further improve control of the gas composition, separated carbondioxide may be purged from the process prior to compression and heatingvia line 94. The heated, compressed carbon dioxide stream is fed fromheat exchanger 92 via line 96 to the primary reformed gas mixture 80 andthe resulting secondary reformer feed stream passed to the secondaryreformer 42 via line 98.

In FIG. 3, the carbon dioxide recovery and recycle stage of FIG. 2 isomitted and heat exchangers 38 and 76 are omitted. The secondhydrocarbon stream by-passing the primary reforming stage via line 36 ismixed with the Fischer-Tropsch tail gas fed via line 32 and theresulting mixture heated in heat exchanger 100 before feeding via line102 to the primary reformed gas 30 to form the secondary reformer feedgas mixture, fed to the secondary reformer via line 104.

The invention is further illustrated by the following calculatedexamples.

EXAMPLE 1

FIG. 4 is a table that contains data calculated for a Fischer-Tropschprocess operated in accordance with the schematic depicted in FIG. 2.The data demonstrates that the process of the present invention is ableto provide a steam ratio in the heat-exchange reactor tubes of 1.25 andthereby, with a nickel steam reforming catalyst avoid carbon deposition,yet based on total hydrocarbon feed to the process, the overall steamratio is 1.0.

EXAMPLE 2

FIG. 5 is a table contains data calculated for a 80,000 barrel-per-dayFischer-Tropsch process operated in accordance with the schematicdepicted in FIG. 3. The data demonstrated that the process of thepresent invention is able to provide a steam ratio in the heat-exchangereactor tubes of 0.88 and thereby, with a precious metal reformingcatalyst avoid carbon deposition, yet based on total hydrocarbon fed tothe process, the overall steam ratio is 0.66.

In the following table the pressure(P, in bar abs.), temperatures (T, in°C.) and flow rates (kmol/h) of the various components of the streamsare quoted, rounded to the nearest integer. 1 bara=10000 Pa or 100 kPa.

1. A process for the production of hydrocarbons comprising: a)subjecting a hydrocarbon feedstock to steam reforming by i) dividing thefeedstock into first and second streams, ii) mixing the first streamwith steam, passing the mixture of the first stream and steam over acatalyst disposed in heated tubes in a heat exchange reformer to form aprimary reformed gas, iii) forming a secondary reformer feed streamcomprising the primary reformed gas and the second hydrocarbon stream,iv) partially combusting the secondary reformer feed stream with anoxygen-containing gas and bringing a resultant partially combusted gastowards equilibrium over a secondary reforming catalyst to form aresultant secondary reformed gas, and v) using the resultant secondaryreformed gas to heat the tubes of the heat exchange reformer, therebyproducing a partially cooled reformed gas, b) further cooling thepartially cooled reformed gas to below the dew point of the steamtherein to condense water and separating condensed water to give ade-watered synthesis gas, c) synthesising hydrocarbons from saidde-watered synthesis gas by the Fischer-Tropsch reaction and separatingat least some of the synthesised hydrocarbons, to give a tail gas, andd) incorporating at least part of said tail gas into the secondaryreformer feed stream before the partial combustion of thereof.
 2. Aprocess according to claim 1 wherein the second hydrocarbon streamcomprises between 5 and 50% by volume of the hydrocarbon feedstock.
 3. Aprocess according to claim 1 wherein carbon dioxide is separated fromthe synthesis gas prior to synthesis of the hydrocarbons and is added tothe secondary reformer feed stream before the partial combustionthereof.
 4. A process according to claim 3 wherein the tail gas andsecond hydrocarbon stream are combined and added to the primary reformedgas separately from the separated carbon dioxide.
 5. A process accordingto claim 1 wherein the de-watered synthesis gas is subjected to a stepof hydrogen separation before it is passed to the Fischer-Tropschhydrocarbon synthesis stage.
 6. A process according to claim 1 whereinthe catalyst disposed in heated tubes in the heat exchange reformercomprises a nickel catalyst and/or a precious metal catalyst.